Network Working Group Daniel Walton
Internet Draft Alvaro Retana
Expiration Date: August 2006 Enke Chen
Cisco Systems
Advertisement of Multiple Paths in BGP
draft-walton-bgp-add-paths-05.txt
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Abstract
In this document we propose a BGP extension that allows the
advertisement of multiple paths for the same address prefix without
the new paths implicitly replacing any previous ones. The essence of
the extension is that each path is identified by a path identifier in
addition to the address prefix.
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1. Introduction
The BGP specification [RFC4271] defines an "Update-Send Process" to
advertise the routes chosen by the Decision Process to other BGP
speakers. No provisions are made to allow the advertisement of
multiple paths for the same address prefix, or Network Layer
Reachability Information (NLRI). In fact, a route with the same NLRI
as a previously advertised route implicitly replaces the previous
advertisement.
In this document we propose a BGP extension that allows the
advertisement of multiple paths for the same address prefix without
the new paths implicitly replacing any previous ones. The essence of
the extension is that each path is identified by a path identifier in
addition to the address prefix.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. How to Identify a Path
As defined in [RFC4271], a path refers to the information reported in
the path attribute field of an UPDATE message. As the procedures
specified in [RFC4271] allow only the advertisement of one path for a
particular address prefix, a path for an address prefix from a BGP
peer can be keyed on the address prefix.
In order for a BGP speaker to advertise multiple paths for the same
address prefix, a new identifier (termed "Path Identifier" hereafter)
needs to be introduced so that a particular path for an address
prefix can be identified by the combination of the address prefix and
the Path Identifier.
Depending on the application and the configuration of a particular
peer, the Path Identifier for a path can be an AS number, or a BGP
Identifier, or an opaque number, with which a path is associated by
the BGP speaker that advertises the path.
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4. Extended NLRI Encodings
In order to carry the Path Identifier in an UPDATE message, the
existing NLRI encodings specified in [RFC4271, RFC2858] are extended
as the following:
+-----------------------------+
| Path Identifier (4 octets) |
+-----------------------------+
| Length (1 octet) |
+-----------------------------+
| Prefix (variable) |
+-----------------------------+
and the NLRI encoding specified in [RFC3107] is extended as the
following:
+-----------------------------+
| Path Identifier (4 octets) |
+-----------------------------+
| Length (1 octet) |
+-----------------------------+
| Label (3 octets) |
+-----------------------------+
.........................
+-----------------------------+
| Prefix (variable) |
+-----------------------------+
The extended NLRI encodings SHALL be used only when the Path
Identifier needs to be carried in an UPDATE message. The usage of the
extended NLRI encodings is specified in the Operation section.
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5. ADD-PATH Capability
The ADD-PATH Capability is a new BGP capability [RFC2842]. The
Capability Code for this capability is specified in the IANA
Considerations section of this document. The Capability Length field
of this capability is variable. The Capability Value field consists
of zero or more of the tuples <AFI, SAFI> as follows:
+------------------------------------------------+
| Address Family Identifier (2 octets) |
+------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+------------------------------------------------+
The meaning and use of the fields are as follows:
Address Family Identifier (AFI):
This field carries the identity of the Network Layer protocol
for which the BGP speaker intends to advertise multiple paths.
Presently defined values for this field are specified in
[IANA-AFI].
Subsequent Address Family Identifier (SAFI):
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute. Presently defined values for this field are
specified in [IANA-SAFI].
When advertising the ADD-PATH Capability to a peer, a BGP speaker
conveys to the peer that the speaker is capable of receiving multiple
paths as well as the single path from the peer for address families
that the speaker supports.
When a tuple <AFI, SAFI> is included in the capability, it indicates
that the BGP speaker intends to advertise multiple paths for the
<AFI, SAFI>. If the ADD-PATH Capability is also received from the
peer, the speaker would then follow the procedures for advertising
multiple paths to the peer for the specified <AFI, SAFI>.
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6. Operation
The Path Identifier specified in the previous section can be used to
advertise multiple paths for the same address prefix without
subsequent advertisements replacing the previous ones. Apart from
the fact that this is now doable, the route advertisement rules of
[RFC4271] are not changed. In particular, a new advertisement for a
given address prefix and a given path identifier replaces a previous
advertisement for the given address prefix and the given path
identifier.
A BGP speaker that has implemented the procedures for receiving
multiple paths MAY advertise the ADD-PATH Capability to its peers
using BGP Capabilities advertisement [RFC2842]. Whether a particular
tuple <AFI, SAFI> is included in the capability depends on the the
configuration, and whether the procedures for advertising multiple
paths for the <AFI, SAFI> are implemented.
A BGP speaker MUST follow the existing procedures in generating an
UPDATE message for a particular <AFI, SAFI> to a peer unless the BGP
speaker advertises the ADD-PATH Capability that includes the <AFI,
SAFI>, and also receives the ADD-PATH Capability from the peer, in
which case the speaker MUST generate a route update for the <AFI,
SAFI> based on the combination of the address prefix and the Path
Identifier, and use the extended NLRI encodings specified in this
document. The peer SHALL act accordingly in processing an UPDATE
message related to a particular <AFI, SAFI>.
7. Applications
The BGP extension specified in this document can be used by a BGP
route reflector [RFC2796] or BGP Confederation ASBR [RFC3065] to
advertise more than just the best path in order to eliminate
persistent route oscillations [RFC3345], or to help achieve optimal
routing in a network.
Other applications are for further study.
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8. Deployment Considerations
The extension proposed in this document provides a mechanism for a
BGP speaker to advertise multiple paths over a BGP session. The
extension is intended for use by only a small number of applications
that require such a functionality. Care needs to be taken in its
deployment to ensure consistent routing and forwarding in a network.
9. IANA Considerations
IANA needs to assign a capability number for the ADD-PATH Capability
described in this document.
10. Security Considerations
This document introduces no new security concerns to BGP or other
specifications referenced in this document.
11. Acknowledgments
This specification combines prior work on "Advertising Multiple
Paths" by Daniel Walton, David Cook, Alvaro Retana, and John Scudder,
with prior work on "Advertisement of the Group Best Paths" by Enke
Chen, and Naiming Shen. The current authors wish to thank all these
authors for their contribution.
Many people have made valuable comments and suggestions, including
Dave Meyer, Srihari Sangli, Eric Rosen, Dan Tappan, Robert Raszuk,
Mark Turner, Danny McPherson and Eugene Kim.
12. Normative References
[RFC4271] Rekhter, Y., T. Li, and S. Hares, Eds., "A Border Gateway
Protocol 4 (BGP-4)," RFC 4271, January 2006.
[RFC2842] Chandra, R. and J. Scudder, "Capabilities Advertisement
with BGP-4," RFC 2842, May 2000.
[RFC2858] Bates, T., R. Chandra, D. Katz and Y. Rekhter,
"Multiprotocol Extensions for BGP-4," RFC 2858, June 2000.
[RFC3107] Rekhter, R. and E. Rosen, "Carrying Label Information in
BGP-4," RFC 3107, May 2001.
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[RFC2796] Bates, T., R. Chandra, and E. Chen "BGP Route Reflection -
An Alternative to Full Mesh IBGP", RFC 2796, April 2000.
[RFC3065] Traina, P., D. McPherson, and J. Scudder, "Autonomous
System Confederations for BGP", RFC 3065, February 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119, BCP 14, March 1997.
[IANA-AFI] http://www.iana.org/assignments/address-family-numbers.
[IANA-SAFI] http://www.iana.org/assignments/safi-namespace.
13. Informative References
[RFC3345] McPherson, D., V. Gill, D. Walton, and A. Retana, "Border
Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC
3345, August 2002.
14. Authors’ Addresses
Daniel Walton
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
Email: dwalton@cisco.com
Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
Email: aretana@cisco.com
Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: enkechen@cisco.com
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